Fluid motors

ABSTRACT

A fluid motor operated by a gas or liquid and having two relatively rotatable parts comprises a plurality of pistons which cooperate with a cam member to produce the rotational output from the motor. The cam member has at least three similar lobes and forms one of the parts and the other part includes at least four cylinders each containing a piston. Each piston/cylinder combination also acts as a slide valve so as to control both the supply of fluid to one of the other pistons and the exhausting of fluid from another of the other pistons.

The present invention relates to fluid motors and more particularly tomotors in which the operating fluid is a gas, such as air.

The invention is particularly concerned with fluid motors in which aplurality of pistons, whose reciprocation is controlled by the fluid,cooperate with a cam member to produce the rotational output from themotor and wherein the pistons also act as valve members controlling thefluid flow. Various designs of such a motor have been proposed, in manyof which the rotor consists of an eccentric or twin lobe cam memberwhich is caused to rotate in response to the actuation of a plurality ofpistons disposed about and associated with the rotor. However, existingdesigns suffer from a number of disadvantages amongst which arerelatively low power output and uneven torque, relatively poor low speedand starting characteristics and undesirably high vibration and noise.

It is an object of the present invention to provide an improved fluidmotor.

The present invention consists in a fluid motor having two relativelyrotatable parts, one of said parts comprising a cam member having atleast three similar lobes and the other of said parts comprising atleast four cylinders each containing a reciprocatable piston, eachpiston and its associated cylinder also acting as a slide valve so as tocontrol the supply of fluid to one of the other pistons and theexhausting of fluid from another of the other pistons, said pistonsproducing a force on said cam member to effect the relative rotation ofsaid parts.

Advantageously separate passages are provided for supplying fluid to thepistons and for exhausting fluid from the pistons and the fluid supplypassages and fluid exhaust passages are similar so that the function ofthe passages can be interchanged thereby enabling the motor to berotated in either direction. Moreover, each cylinder piston combinationis preferably constructed so as to provide a controlled leak or bleedpast the piston for the reasons which will be hereinafter explained.

In one construction according to the invention the pistons are disposedradially around the longitudinal axis of the cam member and one end ofeach piston is connected to a cam follower which engages with the cammember.

In an alternative construction the cylinders are disposed around andparallel to a central axis of the motor and a cam member cooperatingwith the pistons is disposed at at least one end of the motor. In such aconstruction the pistons may be double-ended, a cam member beingdisposed at each of two opposite ends of the motor.

Preferably each cylinder is formed with two groups of ports each ofwhich cooperates with a separate cavity in the associated piston so asto form two slide valves controlling respectively the supply andexhausting of the fluid.

In a preferred construction the motor comprises a body including thecylinders and has fluid supply and exhaust passages formed as channelsin at least one surface of the body and communicating with the ports inthe cylinders, and at least one cover is secured to the body to coverthe channels and thereby form enclosed passages.

In one specific form of the motor the body is generally cylindrical andhas the channels formed in its end surfaces and covered by end plates.In another form of the motor the body is a cylinder having the channelsformed in the annular cylinder surface and covered by a cylindricalsleeve.

The invention also provides a method of operating a fluid motor as abovedescribed which consists in applying fluid simultaneously to both thefluid supply and exhaust passages so as to brake the motor and hold itagainst rotation.

The invention further provides the method of operating a fluid motor asabove described which consists in removing the fluid supply whereby thepistons are allowed to move out of contact with the cam member and hencethe rotatable part of the motor can rotate freely until the fluid supplyis restored and wherein fluid is bled past the pistons to bring adjacentpistons back into contact with the cam member.

The motor is preferably constructed with the cam member mounted as arotor, for example mounted on a shaft which forms the rotational outputof the motor, and the cylinders formed in a stator disposed about theaxis of rotation of the rotor.

The invention will now be further described, by way of example, withreference to the accompanying drawings in which:

FIG. 1 is an exploded perspective view of one embodiment of a fluidmotor according to the invention,

FIG. 2 is a sectional view on a larger scale of a piston and cylinderconstruction,

FIGS. 3A,3B and 3C are diagrams illustrating the mode of operation ofthe motor shown in FIG. 1, and

FIG. 4 is an exploded perspective view of a second embodiment of fluidmotor according to the invention.

Referring to FIGS. 1 and 2, the motor to be described is intendedprimarily as an air motor working from a supply of compressed air. Themotor consists essentially of a rotor indicated at R and a statorindicated at S. The rotor comprises a tri-lobed cam 1 comprising threesimilar lobes mounted on a shaft 2. The ends of the shaft 2 arejournalled in bearings 3 located in bosses 4 attached to the end covers5 for the stator S by means of bolts 6.

The stator S comprises a generally cylindrical structure having fourcylinder cavities disposed radially about the rotational axis of therotor R which is located within the bore 8 of the stator. Each cylindercavity 7 contains a cylinder liner 9 which in turn houses a piston 10,and each cylinder is closed by a cylinder head member 11 secured bybolts 12 to the stator S. The inner end of each piston is provided witha cam follower in the form of a roller 13 mounted on a spindle 14 andwhich engages the lobed surface of the cam 1 of the rotor R. Appropriateseals 15 are provided between the piston 10 and the cylinder linear 9.Diametrically opposite sides of each piston 10 are provided with arecess 16 which cooperates with a pair of rectangular ports 17 formed atdiametrically opposite positions in the wall of the associated cylinderliner 9 so as to form a pair of slide valves to control the supply offluid to and exhaustion of fluid from the rear ends of adjacent pistons.This is effected via the ports 19 in the cylinder liners 9 and thepassages, such as 20, formed as channels in the side walls of the statorS. These channels are formed into closed passages by the end covers 5when they are secured in position on the stator S.

The configuration of the supply and exhaust passages is such that thesupply of fluid to the rear end of any one of the pistons to urge itradially inwards is controlled by means of the slide valve in thefollowing piston in the direction of rotation of the rotor, and theexhaustion of fluid from the cylinder as that one piston is radiallyretracted is controlled by means of the slide valve in the precedingpiston taken in the direction of rotation. The supply of fluid to andexhaustion of fluid from the motor is achieved through connections tothe stator which are not shown.

The operation of the motor mechanism just described will now beexplained with reference to the diagrams of FIGS. 3A, 3B and 3C, inwhich many of the same reference numerals have been used forcorresponding parts.

These Figures show the complete configuration of the fluid supplypassages 19S,20S and fluid exhaust passages 19E, 20E in conjunction withthe associated slide valves VS and VE formed by the recesses 16 andports 17 of the pistons 10 and cylinders 9.

The rotor 1 will be assumed to be rotating clockwise in the direction ofthe arrow X.

For ease of description the four cylinder and piston combinations 9,10bear the respective suffix A,B,C and D.

Referring to FIG. 3A, piston 10A is shown as being supplied withcompressed fluid to its rear end via the main supply line 20S, slidevalve VS of piston 10B and line 19S connected to the rear of cylinder9A. Thus the cam follower 13 of piston 10A is urged against the flank ofthe lobe A of cam 1 to rotate the cam in the clockwise direction. At thesame time, pistons 10B and 10D are floating (since their cylinders arenot connected to either the fluid supply or exhaust) and fluid is beingexhausted from the rear of piston 10C via associated line 19E and theslide valve VE in piston 10B.

FIG. 3B shows the conditions when the piston 10A has almost completedits forward stroke and rotated the cam through some 15°. The supply offluid is now being cut off from the piston 10A by gradual closing of theslide valve VS in the piston 10B as this piston retracts and theexhaustion of the fluid from the rear of the pistons 10C is also beingcompleted via the gradual closing of slide valve VE in piston 10B. Thenext piston to provide a working stroke will be piston 10D acting onlobe B of the cam 1 and, as can be seen, compressed fluid from line 20Sis already being admitted to the rear of this piston via partially openslide valve VS in piston 10A, whilst the rear of piston 10B opposite topiston 10D is being vented to exhaust line 20E through slide valve VE ofpiston 10A.

FIG. 3C shows a further stage in the rotation of the motor when thesupply of fluid to the piston 10A is cut off, since it is at the end ofits working stroke, and fluid is being fully supplied to the piston 10Dthrough the slide valve VS in the piston 10A and exhausted from thepiston 10B through the slide valve VE in the piston 10A. In thisposition the pistons 10A and 10C are floating. The cam 1 has now rotatedthrough 30°. It will thus be seen that in this embodiment employing athree-lobed cam and four pistons, twelve discrete 30° positions for therotor can be obtained.

It will also be understood that the sequence of operations as describedin relation to the working stroke of piston 10A is repeated cyclicallyfor each of the pistons 10B to 10D, thereby causing rotation of therotor by sequential radially inward working strokes of the pistonsacting on the lobes A, B and C of the cam 1.

FIG. 4 shows a further embodiment of motor according to the inventionwhich is constructed so that the piston and cylinder combinations extendaxially parallel to the rotatable shaft of the motor. Two pistons areprovided in each cylinder and respectively cooperate with a cam memberat the corresponding end of the motor to produce rotation of the shaft.

More specifically the motor comprises a stator S housing eight axiallyextending cylinders 107 each of which contains a cylinder liner 109 andtwo pistons 110A and 110B. The stator surrounds a shaft 102 mounted inbearings 103 fitted into the stator; and two rotors 101A,101B, each inthe form of a six lobed cam member are attached one at each end of theshaft 102 and secured by means of a key 102A and a threaded collar 106.The fluid supply and exhaust passages are formed as channels 120 in theouter cylindrical surface of the stator S and are closed by a tubularcover sleeve 105 extending over the stator S. The motor operatesgenerally in the same manner as has been described with reference toFIGS. 1 to 3.

In both of the constructions described, a controlled bleed or leak isprovided past the slide valve around the surface of the piston.Alternatively this controlled bleed could be formed by a small holethrough the piston or a flattened area or groove on the surface of thepiston in the vicinity of the recesses 16, as shown at 16A in FIG. 2.Such an arrangement enables the motor to be run without fluid powersupplied and with all the pistons out of contact with the rotor, forexample when it is desired to rotate the rotor by hand. With such anarrangement the rotor can be rotated freely without any resistance beingcaused on the rotor, for example by springs urging the pistons intocontact with the rotor, since no such springs are needed.

Moreover, by providing separate fluid supply and exhaust passages toeach piston and a control bleed around the piston from the supply to theexhaust side it is always possible to urge one piston into contact withthe cam of the rotor even from a "stalled position" and hence to startthe motor without difficulty. On the contrary, prior art constructionsalways need to provide springs to urge the pistons against the rotor inorder to prevent a permanently stalled condition.

Because the springs can be eliminated, the motor can spin freely whenall the pistons are retracted and no power is on the motor whereby therotor offers little resistance to manual manipulation.

Moreover, the use of individual inlet and outlet ports for each cylindergives a fast response time to the motor on starting and stopping.

The constructions of fluid motor according to the invention provide themotor with a high starting torque, a substantially stepless speedcontrol with little torque fluctuation, the ability to be stalled at apredetermined torque and a good tolerance to intermittent starting andreversing. The motor can also act as a brake by supplying fluid to boththe supply and exhaust passages simultaneously.

In order to achieve the advantages of the present invention overpreviously proposed designs employing a single lobe or bi-lobed cam, itis essential that the motor should employ a cam member having at leastthree lobes in combination with at least four pistons. Such anarrangement provides at least twelve working strokes per revolution ofthe rotor that is to say a new power stroke is implemented for every 30degrees of shaft rotation which leads to smooth torque transferance,reduced cyclic torque variation and improved starting torquecharacteristics. In addition, for a given power rating a three-lobedrotor will experience shorter piston strokes with reduced vibration andnoise generation and produce better acceleration. The motor can alsoreadily be made bi-directional and is very suitable for continuouslyreversing operations.

Furthermore, the motor has very good stepping characteristics withthirty degree steps in either direction and if employed in conjunctionwith a high reduction gearbox connected to its output, the resolutionobtained would be sufficiently good to make the motor suitable forprecise positional control applications, particularly where thesuitability of air as the power supply and control media has advantagesover other prime sources such as electricity or hydraulic fluid.

Whilst particular embodiments are being described it will be understoodthat various modifications may be made without departing from the scopeof this invention. Thus, whilst the motor has been described as having acylinder block as the stator and the cam member or members as the rotor,reverse constructions could also be devised in which the pistons weremounted in the rotational part of the motor and the cam member formedpart of the stator.

The motor according to the present invention also lends itself to easymanufacture by means of well established machining and/or castingtechniques without necessitating expensive and difficult machiningoperations or requiring complex and difficult casting techniques. Themotor may be constructed mainly from metals, plastics materials or acombination of both materials.

It is also to be understood that whilst the motor has been primarilydescribed as working from a gaseous fluid such as air, it may also bedesigned to work from a hydraulic fluid such as oil.

Moreover, the tri-lobed rotor shown may be of other specific shape,although its shape is preferably such as to produce a high harmonicpower output, that is to say a continuous power output having relativelysmall fluctuations.

I claim:
 1. A fluid motor having two relatively rotatable parts, one ofsaid parts comprising a cam member having at least three similar lobesand the other of said parts having at least four cylinders, areciprocatable piston located in each cylinder, two separate oppositelydisposed cavities in the wall of each piston, two pairs of ports in thewall of each cylinder, each one of said pairs of ports cooperating withone of said cavities thereby to provide two individual slide valves,said two slide valves operating simultaneously with the reciprocation ofthe piston such that one slide valve controls the supply of fluid to apreceding piston in the direction of rotation of the motor and the otherslide valve controls the exhaustion of fluid from a succeeding piston insaid direction of rotation, the rotation of said motor being effected bythe force of said pistons acting on said cam member.
 2. A fluid motor asclaimed in claim 1, in which the fluid supply passages and the fluidexhaust passages of the motor are similar such that the function of thepassages can be interchanged thereby enabling the motor to be rotatablydriven in either direction.
 3. A fluid motor as claimed in claim 1, inwhich each cylinder and piston combination is constructed so as toprovide a controlled leak or bleed past the piston.
 4. A fluid motor asclaimed in claim 1, in which the pistons are disposed radially aroundthe axis of the cam member.
 5. A fluid motor as claimed in claim 1, inwhich the cylinders are disposed around and parallel to a central axisof the motor and a cam member cooperating with said pistons is disposedat at least one end of the motor.
 6. A fluid motor as claimed in claim5, in which each cylinder contains two pistons and a cam member isdisposed at each of the two opposite ends of the motor.
 7. A fluid motoras claimed in claim 1, in which one end of each piston is connected to acam follower which engages with the cam member.
 8. A fluid motor havingtwo relatively rotatable parts, one of said parts comprising a rotatablecam member having at least three similar lobes and the other of saidparts comprising a stationary body member incorporating at least fourcylinders, said cylinders being disposed about the axis of rotation ofsaid cam member and each cylinder being diametrically opposite anothercylinder, a reciprocatable piston located in each cylinder, one end ofeach piston being engageable with the cam member and the other end ofeach piston being free in said cylinder, two separate diametricallydisposed cavities in the side wall of each piston, two pairs of ports inthe wall of each cylinder, each one of said pairs of ports cooperatingwith one of said cavities thereby to provide two individual slidevalves, said two slide valves operating simultaneously with thereciprocation of the piston such that one slide valve controls thesupply of fluid to a preceding piston in the direction of rotation ofthe rotor and the other slide valve controls the exhaustion of fluidfrom a succeeding piston in said direction of rotation, the rotation ofsaid rotor being effected by the force of said pistons acting on saidcam member.
 9. A fluid motor as claimed in claim 8, comprising a bodyhaving bores forming the cylinders and further having fluid supply andexhaust passages formed as channels in at least one surface of said bodyand at least one cover secured to said body to cover said channels andthereby form enclosed passages.